The invention relates to a method for adapting a lambda control of an internal combustion engine having an exhaust gas probe disposed in an exhaust gas duct as part of an exhaust gas monitoring system, wherein the adaptation is carried out on the basis of a comparison of a modeled and a measured signal according to a predetermined change in an fuel-air ratio of an air-fuel mixture supplied to the internal combustion engine and wherein the measured signal is an actual value of an output signal of the exhaust gas probe and the modeled signal is a model value, which is derived from the air-fuel mixture supplied to the internal combustion engine using an exhaust gas model.
The invention further relates to a device for carrying out the method.
In order to reduce harmful exhaust emissions in passenger cars with Otto engines, 3-way catalytic converters are used as emission control systems, which only then sufficiently convert exhaust gases if the air-fuel ratio λ is very precisely adjusted. For this purpose, the air-fuel ratio λ is measured by an exhaust gas probe disposed upstream of the emission control system. The storage capacity of such an emission control system for oxygen is utilized for the purpose of receiving oxygen during lean phases and in turn dispensing oxygen during rich phases. This ensures that oxidizable constituents of harmful gas contained in the exhaust gas can be converted. An exhaust gas probe disposed downstream of the emission control system thereby serves to monitor the oxygen storage capacity of said emission control system. The oxygen storage capacity has to be monitored within the scope of the on-board diagnostics because said capacity represents a measurement for the conversion capacity of said emission control system. In order to determine said oxygen storage capacity, either the emission control system is initially saturated with oxygen in a lean phase and is subsequently emptied in a rich phase having an exhaust gas of a known lambda ratio with regard to the amount of exhaust gas passing through said emission control system or said system is initially emptied of oxygen in a rich phase and in a lean phase is subsequently filled with an exhaust gas of a known lambda ratio with regard to the amount of exhaust gas passing through said system. The lean phase is ended if the exhaust gas probe downstream of said emission control system detects the oxygen, which can no longer be stored by said emission control system. A rich phase is likewise ended if said exhaust gas probe detects the passage of rich exhaust gas. The oxygen storage capacity of the emission control system corresponds to the amount of reducing agent supplied during the rich phase to empty said system of oxygen or respectively to the amount of oxygen supplied during the lean phase to fill said system with oxygen. The exact amounts are calculated from the signal of the exhaust gas probe disposed upstream of said emission control system and from the exhaust gas mass flow determined from other sensor signals.
If the dynamic behavior of the exhaust gas probe disposed upstream of the emission control system decreases, e.g. as a result of contamination or ageing, the air-fuel ratio can no longer be adjusted with the required precision so that the conversion capacity of said emission control system decreases. In addition, deviations during the diagnosis of said emission control system can result which can lead to an emission control system which is in good working order to be mistakenly determined to be inoperative. Lawmakers require a diagnosis of the exhaust gas probe properties during vehicle operation in order to ensure that the required air-fuel ratio can still be adjusted with sufficient accuracy, to ensure that the admissible limit values are not exceeded and to ensure that said emission control system is being correctly monitored. Among other things, a degradation of the probe dynamics must be detected, which can become apparent as a result of an enlarged time constant and/or dead time.
The German patent publication DE 10 2008 042 549 A1 discloses a method and device for diagnosing the slew rate and the dead time of an exhaust gas probe, which is disposed in the exhaust gas duct of an internal combustion engine, wherein the diagnosis is carried out on the basis of a comparison of a modeled and a measured signal after a predetermined change in a fuel-air ratio of an air-fuel mixture delivered to the internal combustion engine has taken place and wherein the signal is an output signal of the exhaust gas probe or a modeled or measured signal derived from the output signal. Provision is thereby made for a first extreme value to be determined in the curve of the modeled signal and for a first point in time and a first starting value to be determined if the modeled signal deviates from the first extreme value by a predetermined amount. Provision is also thereby made for a second extreme value in the curve of the measured signal to be determined. In addition, provision is made for a second point in time and a second starting value to be determined if the measured signal deviates from the second extreme value by a predetermined value, for a first integral for a predetermined period of time, beginning at the first point in time, over the difference between the first starting value and the modeled signal to be formed and for a second integral for a second period of time, beginning at the second point in time, over the difference between the second starting value and the measured signal to be formed, for the second period of time to equal the predetermined period of time and for the end of the second period of time to be determined on the basis of the point in time of the change in the fuel-air ratio or on the basis of the first point in time. Provision is finally made for a quantitative comparison value to be formed from a quantitative comparison between the first integral and the second integral, the slew rate and/or the dead time of the exhaust gas probe being suggested from said quantitative comparison value.
This method uses steplike adjustments of the air-fuel ratio, by means of which the dynamic behavior of the exhaust gas probe is evaluated. In so doing, an anisotropy, i.e. from rich to lean or from lean to rich, is additionally distinguished. For this purpose, the area below the lambda signal of the exhaust gas probe is integrated for a certain period of time after the step change and set in relation to an analogously calculated area on a lambda signal modeled in the control device. If the calculated ratio is smaller than an applicable threshold, the exhaust gas probe no longer meets the required level of dynamic behavior.
In order to model the air-fuel ratio in the control device, a first-order filter having a time constant T, a gain K=1 as well as a dead time model having the dead time Tt is used. The first-order filter can thus be described as follows:G(S)=Kexp(−Tts)/(Ts+1)  (1)
In some engines, the method proves however to be partially insufficiently robust and leads to dispersive diagnostic results. A reason for this is among other things that the real gain of the lambda controlled system frequently deviates from the value K=1, which is theoretically expected and used in the control device. This has a strong influence on the area ratio and is therefore misinterpreted as a change in the probe dynamics although said deviation relates to a deviating loop gain. This behavior is known for all engines to a greater or lesser extent, wherein the gain from operating point to operating point varies without a systematic basis and can be compensated only with difficulty.
A further method—referred to in this case as the gradient method—uses likewise steplike adjustments of the air-fuel ratio and determines the maximum gradient according to amount of the measured air-fuel ratio within a certain period of time after the step change has taken place. The period of time for evaluation results from an applicable deviation, by which the measured air-fuel ratio may change after the step change. Said period of time is interpreted as the dead time, corrected by the difference between the point in time at the end of the evaluation and the point in time whereat a straight line having the previously determined maximum gradient divided by the value of the measured air-fuel ratio at the end of the evaluation intersects with the value of the minimum/maximum measured air-fuel ratio during the evaluation. A time constant error is detected by a maximum gradient which is too small in amount.
The same calculation can also be carried out analogously with inverted lambda signals.